c. The refractory cast, with the slotted end of the sprue down, is rested on a pouring spout from a duplicating flask or an inlay ring and returned to the glass slab. The cast with the painted layer of investment should not be set on a bench where it may be affected by vibration. After this layer of investment has set, it is ready to be full-flasked (B, fig 10-51).

d. Chrome-base alloy flask sets contain a selection of stainless steel flask formers. A flask is selected which maintains 8 to 12 mm. clearance of the cast from the sides of the flask (C, fig 10-51). The investment material is mixed according to the manufacturer's directions. The flask is placed on a plastic square or a stainless steel bench top and, using a minimum of vibration, the flask is about three-fourths filled (D, fig 10-51). Enough vibration is used to settle the investment material into the flask, but not enough to remove air bubbles. Before the refractory cast is inserted into the flask, it is dipped in water. Excess water is blown off. This keeps the layer of painted investment material from extracting water from the investment mix.

e. The position of the refractory cast in the flask must be determined when the overjet technique is used. A single sprue lead should always be toward the seam. If an auxiliary sprue is used in addition to the main sprue lead, the main lead should face toward the seam. If two sprue leads of equal thickness are used, the cast is centered in the flask so that both leads point as nearly as possible toward the seam. Using this method, the thinner areas of the pattern will always be far

thest from the seam, the centrifugal force of the casting machine will aid the flow of the metal, and every casting will be made in the same relative position. If a single sprue either from the anterior or posterior direction is used, as in full palate cases, the wax pattern will always face the seam.

f. The dental prosthetic specialist teases the cast into the flask, being careful not to create voids. The cast is centered in the flask as accurately as possible. There should not be more than 6 to 8 mm. distance from the bottom of the flask to the painted layer of investment. The refractory cast should displace enough investment material so that the flask will be full. If more material is required, it can easily be added (E, fig 10-51).

g. After the investment has reached the final set, the end of the flask is squared by rubbing the mold over a piece of screen wire or sandpaper.

h. When a stainless steel sprue cone is used, it must be removed before the mold is placed in the wax elimination furnace. The sprue is removed by inserting a screwdriver in the slot and twisting. The sprue should drop out easily.

i. When the flask former makes contact with the screening, the retaining clip is removed and the flask former slid off the investment mold. Loose particles of investment material are blown out of the sprue hole with a blast of air. The ingot size is marked on the bottom of the mold with jeweler's rouge (F, fig-10-51).

j. Advantages of the double investing technique are that it prevents loss of the casting if the mold

splits or cracks, decreases the chance of trapping air bubbles on wax pattern, and makes possible a more permeable mold.

10-39. The Burnout

a. Loading the Oven (fig 10-52). The following principles apply to loading the oven:

(1) Sprue holes are faced down so that the wax can be eliminated more easily from the mold.

(2) Molds must not touch the oven walls or any inside component of the oven, especially the thermocouple. There must be moisture in the mold so that steam will produce a uniform heat saturation during the initial burnout. This minimizes the chances of the mold cracking.

(3) When two layers of molds are loaded in the oven, the top layer is staggered. Thus, the wax being eliminated from the sprue holes will drain between the molds on the bottom layer.

(4) A well-ventilated oven is required to produce a successful burnout.

Figure 10-52. Loading the oven.

b. Procedure. The molds are placed in a cold oven. The temperature is raised to 1350° F. over 2 to 3 hours. The molds are heat-soaked 1 hour at 1350° F. Total burnout time should be 3 to 4 hours.

10-40. The Casting Procedure

a. The furnace should be loaded in a definite pattern and a record made so that the ingot size will be known before the mold is removed from the furnace.

b. Large molds should be loaded first so they will cast last. This allows more time for heatsoaking of large molds and reduces the number of changes in balancing the casting machine.

c. Various types of casting machines are used for casting chrome-base alloys. The manufacturer's directions must always be followed. After the casting has been done, the mold is set aside to cool 20 to 30 minutes. By then it should be cool enough to handle. The mold should never be quenched in water. This can cause serious warpage of the casting.

10-41. Removing the Investment

a. The side of the mold is tapped lightly with a plaster knife. This permits the outer investment to fall away easily, leaving the painted layer of investment and the main sprue reservoir exposed.

b. The reservoir is held with pliers and the sprue tapped with a hammer (A, fig 10-53). Most of the remaining investment will fall away from the casting (B, fig 10-53).

c. The casting is sandblasted to remove the remaining investment (C, fig 10-53).

10-42. Finishing the Framework

a. Vapor Blast (Liquid Honing) Technique. The framework is placed in a vapor blast (liquid honing) machine (fig 10-54) to start the finishing stage of framework polishing. Wherever possible, the sprue is cut by using a large separating disk. Smaller disks can be used for places that are inaccessible to large disks. A heatless stone or large separating disk is used to remove the bulk of metal where the sprue was attached and to do all rough grinding. A mounted point is used for shaping the casting. Small points of various shapes and finer grits are used for final shaping. A stone is not used on stippled external areas or areas which will lie against tissue in the mouth.

b. Electrolytic Polishing Technique. After the sprue is removed, the framework is placed in an electrolytic polisher (fig 10-55). This is the only polishing that is applied to rugae and tissuecontact areas. It includes the following steps:

(1) The casting is rinsed in water to remove any fine grit clinging to scratches in the metal. The casting is then dried thoroughly.

(2) The bowl of electrolyte polishing solution is heated in a pan of water until the temperature reaches 120° F. For castings with extensive coverage or deep-vaulted palates, the solution is used at room temperature.

(3) The bowl is removed from the pan. Cathode clip "C" is attached to the terminal on the assembly. Anode clip "A" is attached to the casting and both are submerged in the solution. Clip "A" is always attached to the posterior portion of palatal castings to prevent the pocketing

of escaping gases. The casting must not touch the cathode assembly and must be submerged completely.

(4) The controller is turned on and regulated to the proper amperage. For each square inch of surface of the casting 12 amperes is used; the surfaces of both sides must be considered. Average cases require between 4 and 7 amperes.

(5) The time clock is set for 6 minutes for average castings when using polishing solution preheated to 120° F. or for 8 to 10 minutes when using polishing solution at room temperature.

(6) When the process is completed, the controller is switched off and the anode assembly removed. The casting is released into the rinsing bowl containing water. The anode clip is rinsed immediately to prevent corrosion.

(7) The polishing solution should be effective for about 200 castings. If the casting does not shine after this process, the dental prosthetic specialist should check the temperature of the solution, insure that the casting was thoroughly sandblasted or vapor-blasted, find out if the proper amperage was used, examine the solution to see if it is contaminated or ineffective, and determine if the casting was processed long enough.

10-43. The Final Polish

a. A rubber wheel in the high speed lathe is used to remove all scratches left on the casting by the abrasive stones. Rubber points or knifeedge wheels are used to reach under leading edges of clasps, insides of clasps, and any other places inaccessible to the rubber wheel.

b. The casting is polished until it has a high luster. Chrome-base alloy low polishing compound is used on a bristle brush, muslin wheel, felt wheel, or felt point. A bristle brush should be used for areas that cannot be reached with a muslin felt wheel or point.

c. Any remaining particles of the polishing compound are removed from the casting by using a heated solution of detergent as a cleansing agent. Either a 5 percent solution of ammonia and green soap or the ultrasonic cleaning machine is used (A, fig 10-56). Next, the casting is dried. The polishing is completed, using a chrome-base alloy high polishing compound on a muslin felt wheel or point. Then the casting is placed in an ultrasonic cleaning machine to remove all remaining particles of the polishing compound.

10-44. General

Gold alloy is used occasionally for removable partial denture frameworks. These frameworks are not as strong or as rigid as chrome-base alloy frameworks. Gold alloy frameworks are more expensive. The procedures, however, are not as complex for gold alloy castings as those used for chrome-base alloy castings nor is special equipment needed. The procedures used to survey, design, blockout, relieve, and duplicate the master cast are the same for gold alloy frameworks as for chrome-base alloy frameworks. The investment material for the refractory cast for a gold alloy casting, however, must be a different type because of the difference in the percentage of shrinkage between the two types of alloys as they cool after casting. To obtain the correct amount of compensating expansion, the proper type of investment material must be used and the manufacturer's instructions followed. Artificial stone duplicate casts and refractory casts are fabricated as described in paragraph 10-29. The wax pattern framework is prepared as described for chromebase alloys, except that slightly thicker patterns are selected because of the lesser structural strength and rigidity of the gold alloy. The methods of spruing described for chrome-base alloy (para 10-35) also apply to the gold alloy framework technique.

10-45. Investing

a. General. The ratio of investment powder to water used in the mix is extremely important; the manufacturer's instructions must be followed exactly to produce the desired expansion. The water must be clean and free of impurities, such as sulfur, which would contaminate the gold. If there is any doubt about the purity of the water, distilled water should be used. Spatulation methods and time are governed by the manufacturer's instructions.

b. Procedures.

(1) Before the investment is mixed, the selected casting ring is lined with one layer of asbestos to provide a cushion for the expansions of the investment. The asbestos strip should be placed so that it leaves about 6 mm. unlined at the end of the ring at which the central sprue will be located. The investment-to-metal contact in this area will prevent the investment from shifting its position in the ring during heating and handling (A, fig 10-57).

(2) The asbestos is moistened so that it will

not absorb water from the investment and change the powder-water ratio. It is not necessary to soak the refractory cast, because the beeswax or rosin dip has sealed its surface and water will not be drawn out of the investment ring.

(3) The fit of the cast in the ring is checked to be sure there is at least 6 mm of clearance between the top of the pattern and the end of the ring (B, fig 10-57). Rather than use an unduly long sprue to place the pattern this close to the end of the ring, the desired result can be obtained better by using a shorter sprue and not completely filling the ring with investment.

(4) With the ring removed, the framework pattern is painted with a wetting agent to reduce surface tension and aid the flow of the investment. (5) The first mix of investment is prepared and painted over the pattern. A small brush is used to push the material until the entire pattern has been covered to a thickness of 3 to 6 mm (C and D, fig 10-57).

(6) After the primary coat of investment has set, it is soaked in water.

(7) If a sprue former has been used, the casting ring is lined with damp asbestos and is placed over the cast and on the sprue former base. A mark may be scratched on the outside of the ring opposite the location of the thinnest component of the pattern. A second mix of investment is poured holding the ring on a vibrator until the ring is filled to the proper level. It is allowed to stand 1 hour.

(8) If a sprue cone is used, the casting ring is filled with a second mix of investment and the cast is lowered upside down to the previously selected depth (E, fig 10-57). The sprue cone must be centered in the ring and the thinnest component of the pattern should be located on the side farthest from the dimple (or scratched mark) on the ring. It is allowed to stand 1 hour. 10-46. Elimination of Wax Pattern (Burnout) a. Principles of Burnout Procedure.

(1) The purpose of the burnout is to eliminate the pattern and the moisture in the investment. A proper burnout leaves the investment free of all residue and provides a quick, easy escape of hot gases from the mold space when the casting is made. In addition, the heating of the investment during burnout expands it, enlarging the mold cavity to provide some compensation for the shrinkage of the gold alloy casting as it cools. (2) A gas or electric furnace is used for the